The homopolymer polyvinylidene fluoride (PVDF) has received much attention as a material for use in the production of hydrophobic polymeric membranes. PVDF has good chemical and thermal resistances, and asymmetric membranes can be produced using PVDF via phase inversion method. Thus membranes produced using PVDF have been used in a number of separation applications including membrane distillation, pervaporation and gas absorption. However problems have been encountered with PVDF membranes, one of which is that the hydrophobicity of PVDF membranes is not sufficiently high which has resulted in them being unable to sustain long term performance.
There has been recent interest in the use of the copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as a membrane material. In contrast to polyvinylidene fluoride (PVDF), PVDF-HFP possesses lower crystallinity and higher free volume due to the incorporation of an amorphous phase of hexafluoropropylene (HFP) into the main constituent vinylidene fluoride (VDF) blocks. Furthermore, the addition of the HFP group increases the fluorine content and makes PVDF-HFP more hydrophobic than PVDF. The strong hydrophobicity of PVDF-HFP makes this material suitable in a range of applications. Thus PVDF-HFP is suitable as a material to make membranes for use in the applications of membrane distillation, pervaporation separation, and membrane contactors.
Most of the work to date on PVDF-HFP membrane preparation has related to flat sheet membranes. For example, a liquid extraction/activation method using a volatile plasticizer was developed by Bellcore to make microporous PVDF-HFP films. Later the phase inversion method with different dope systems was adopted to cast PVDF-HFP flat sheet membranes. Further work led to the proposal that carbon dioxide be used as the non-solvent for ease of solvent recovery during the phase inversion of PVDF-HFP flat sheet membrane preparation.
However there have been few reports on the preparation of PVDF-HFP hollow fiber membranes. This may be due to the fact that PVDF-HFP is generally not intended for use as a membrane material for separation purposes. In addition, hollow fiber fabrication is a more complicated process involving various factors including phase inversion kinetics and two coagulation processes. In contrast to flat sheet configuration, an important advantage of hollow fiber membranes is that hollow fibers can be used to form compact modules with very high membrane surface areas, which is favourable for practical applications.
In work leading up to the invention, the inventors successfully fabricated PVDF-HFP asymmetric microporous hollow fiber membranes using a non-solvent induced phase inversion method. However it was found that the pure water flux of the hollow fiber membranes made by PVDF-HFP/N-Methyl-2-pyrrolidone (NMP) dope solutions without an additive was quite small, even though a low polymer concentration (15 weight %) was used. The addition of polyvinylpyrrolidone) (PVP) as an additive into the dope solution was tested which was believed to promote the formation of macrovoids in the membrane, and the pure water flux was thus increased. However there was a need to further improve the structure and performance of the membranes produced. It was believed that by using a suitable additive as a pore former to adjust the membrane structure, the permeation performance of the hollow fiber membranes could be improved.
It was against the above background that the present invention has been devised.
References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Singapore or elsewhere.